Concentrations of 239Pu and 240Pu and Their Isotopic Ratios Determined by ICP-MS in Soils Collected from the Chernobyl 30-km Zone

Soil samples collected from three forest sites within the 30-km zone around the Chernobyl reactor were analyzed for 239 Pu and 240 Pu by ICP-MS. The average 240 Pu/ 239 Pu atom ratio in contaminated surface soil samples, values of which are scarce in the literature, was 0.408. There were almost no differences in the 240 Pu/ 239 Pu ratios between the individual samples analyzed, although the 239+240 Pu levels varied very widely (i.e. from 6.3 to 1430 Bq kg - 1 dry weight) depending on the distance from the reactor and on the soil layers investigated. This result corresponded to area-related activities for 239+240 Pu between 1.1 kBq m - 2 and 13.3 kBq m - 2 . It was estimated that about half of the Pu migrated from the organic layers to the underlying mineral layers. The 240 Pu/ 239 Pu ratio observed in the Chernobyl area was much higher than that attributed to weapons fallout (ca. 0.18). The high ratio was related to the high burn-up grade of the reactor fuel. The 240 Pu/ 239 Pu ratio observed might be used as a “fingerprint” in identifying the distribution of Chernobyl-derived Pu in the environment and in distinguishing it from other sources, e.g. global fallout. Relationships between the concentrations of Pu and those of 137 Cs, 60 Co, and 125 Sb were also discussed.